It is a convention. The metallic elements have what is known as a metallic bond that consists of the atoms are united by a current or "sea" of electrons which are free to circulate in all the "piece" of metal.
In strict sense, there is not such thing as a molecule of sodium or iron, but all the material is bound by that sea of electrons that was mentioned. Thus, for the purposes of chemical reactions the metal element is counted as an individual atom, i.e. monatomic.